On Fri, Sep 25, 2020 at 1:45 PM Ralph Campbell <rcampbell@xxxxxxxxxx> wrote:
>
> There are several places where ZONE_DEVICE struct pages assume a reference
> count == 1 means the page is idle and free. Instead of open coding this,
> add a helper function to hide this detail.
>
> Signed-off-by: Ralph Campbell <rcampbell@xxxxxxxxxx>
> ---
> fs/dax.c | 8 ++++----
> fs/ext4/inode.c | 2 +-
> fs/xfs/xfs_file.c | 2 +-
> include/linux/dax.h | 5 +++++
> 4 files changed, 11 insertions(+), 6 deletions(-)
>
> diff --git a/fs/dax.c b/fs/dax.c
> index 994ab66a9907..8eddbcc0e149 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -358,7 +358,7 @@ static void dax_disassociate_entry(void *entry, struct address_space *mapping,
> for_each_mapped_pfn(entry, pfn) {
> struct page *page = pfn_to_page(pfn);
>
> - WARN_ON_ONCE(trunc && page_ref_count(page) > 1);
> + WARN_ON_ONCE(trunc && !dax_layout_is_idle_page(page));
> WARN_ON_ONCE(page->mapping && page->mapping != mapping);
> page->mapping = NULL;
> page->index = 0;
> @@ -372,7 +372,7 @@ static struct page *dax_busy_page(void *entry)
> for_each_mapped_pfn(entry, pfn) {
> struct page *page = pfn_to_page(pfn);
>
> - if (page_ref_count(page) > 1)
> + if (!dax_layout_is_idle_page(page))
> return page;
> }
> return NULL;
> @@ -560,11 +560,11 @@ static void *grab_mapping_entry(struct xa_state *xas,
>
> /**
> * dax_layout_busy_page - find first pinned page in @mapping
> - * @mapping: address space to scan for a page with ref count > 1
> + * @mapping: address space to scan for a page with ref count > 0
> *
> * DAX requires ZONE_DEVICE mapped pages. These pages are never
> * 'onlined' to the page allocator so they are considered idle when
> - * page->count == 1. A filesystem uses this interface to determine if
> + * page->count == 0. A filesystem uses this interface to determine if
> * any page in the mapping is busy, i.e. for DMA, or other
> * get_user_pages() usages.
> *
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index bf596467c234..d9f8ad55523a 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -3927,7 +3927,7 @@ int ext4_break_layouts(struct inode *inode)
> return 0;
>
> error = ___wait_var_event(&page->_refcount,
> - atomic_read(&page->_refcount) == 1,
> + dax_layout_is_idle_page(page),
> TASK_INTERRUPTIBLE, 0, 0,
> ext4_wait_dax_page(ei));
> } while (error == 0);
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index a29f78a663ca..29ab96541bc1 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -750,7 +750,7 @@ xfs_break_dax_layouts(
>
> *retry = true;
> return ___wait_var_event(&page->_refcount,
> - atomic_read(&page->_refcount) == 1, TASK_INTERRUPTIBLE,
> + dax_layout_is_idle_page(page), TASK_INTERRUPTIBLE,
> 0, 0, xfs_wait_dax_page(inode));
> }
>
> diff --git a/include/linux/dax.h b/include/linux/dax.h
> index 43b39ab9de1a..3f78ed78d1d6 100644
> --- a/include/linux/dax.h
> +++ b/include/linux/dax.h
> @@ -238,4 +238,9 @@ static inline bool dax_mapping(struct address_space *mapping)
> return mapping->host && IS_DAX(mapping->host);
> }
>
> +static inline bool dax_layout_is_idle_page(struct page *page)
> +{
> + return page_ref_count(page) <= 1;
Why convert the check from "== 1" to "<= 1" and then back to the ==
operator in the next patch? A refcount < 1 in this path before your
other change is a bug.
